Communication terminals manufactured in a portable form have been used in a great variety of fields due to their convenience in use and good portability. Recently such communication terminals have developed in the form of smart phone having various user functions, thus offering much convenience and great entertainment.
Meanwhile, users who have several terminals as well as a smart phone have had interest in sharing data stored in respective terminals. Additionally, the popularization of smart phones today gives rise to a growing interest in sharing data among users. For these reasons, a cloud service begins to again grab the attention of the market.
A conventional cloud service performs an authentication process on the basis of ID and password which have been registered in advance for access from terminals. However, ID and password may be easily spilled, and thus worry about security of a cloud service is increasing. Particularly, since a cloud service tends to be used for saving personal information, the issue of information spill becomes more serious. Therefore, more enhanced technique that has ability to reliably support a data sharing of users without worry about information spill is required.